This invention relates to electrochemical cells, particularly to additives for the anodes, cathodes and/or electrolytes of such cells for the purpose of enhancing electrochemical performance of the cells.
Battery manufacturers are continually trying to improve the electrochemical performance of their batteries, since even relatively small improvements of performance characteristics such as discharge capacity, cycle life and shelf life can provide advantages in a highly competitive market. At the same time, the current requirements of electronic devices that are powered by batteries are increasing, and the capacity at higher discharge rates is becoming a more important characteristic for these batteries. This is particularly true of both primary and rechargeable alkaline batteries.
One approach that has been used to improve the discharge capacity and/or cycle life of electrochemical cells is to use performance enhancing additives in the negative electrode, positive electrode and/or electrolyte. Such additives may affect these improvements by reducing the internal resistance of the cells, increasing ionic conductivity, preventing the formation of undesirable byproducts, and so forth.
Evans (U.S. Pat. No. 4,465,747) discloses a nonaqueous cell with an organic solvent electrolyte and a cathode containing MnO2 and a minor amount of an additive selected from the group consisting of borates, silicates, molybdates, phosphates, aluminates, niobates, tantalates, titanates, vanadates, zirconates, manganates, cobaltates and tungstenates of alkali metals or alkaline earth metals. Langan (U.S. Pat. No. 4,478,921) discloses a nonaqueous cell with a cathode containing MnO2 and a minor amount of MnCO3. The purpose of these additives is to minimize the increase in internal impedance of the cell during storage or discharge. The additive is believed to prevent degradation of the electrolyte caused by reaction of the electrolyte with surface acidic groups on the MnO2 or incompletely neutralized salts used in preparing the electrolyte solution.
Taucher et al. (PCT Patent Publication No. WO 93/12551) disclose primary and rechargeable alkaline Zn/MnO2 cells with barium compounds (e.g., oxide, hydroxide or sulfate) added to the cathode to improve cell capacity. The barium compound slows down the formation of hetaerolite, which cannot participate in the cycling process and tends to expand and destroy the cathode structure. Cathode additives for improving the capacity of alkaline Zn/MnO2 cells have been disclosed. Swierbut et al. disclose the use of one or more of SnO2, Fe2O3xe2x80x94TiO2, TiO2 (P-25), BaTiO3, K2TiO3, Nb2O5 and SnO in U.S. Pat. No. 5,599,644. Nardi et al. disclose the use of at least one of SrTiO3 or CoTiO3 in U.S. Pat. No. 5,895,734. CaWO4, MgTiO3, BaTiO3, CaTiO3, ZnMn2O4 and Bi12TiO20 are disclosed by Davis et al. in U.S. Pat. No. 5,532,085. Mieczkowska et al. disclose the use of Bi2O3, PbO2, SnO2, Co3O4, CoO, Bi2O3.3ZrO3 and K2Cr2O7 in U.S. Pat. No. 5,516,604. In U.S. Pat. No. 5,342,712, Mieczkowska et al. teach that the useful service life of primary alkaline Zn/MnO2 cells can be extended as a result of increased mobility of ionic flow during discharge by adding anatase TiO2 to the cathode.
While there may be advantages to using such additives, in many cases the advantages can be at least partially offset, particularly on heavy drains, by the relatively high resistivities of the additives which increase the internal resistance of the cell.
It is known that the physical and electrochemical properties of semiconductor materials such as SiO2, TiO2 and SnO2 can be modified by doping those materials with other cations. This approach has been used to modify materials for use in the fields of semiconductors, photoelectrochemistry and solid state sensors. For example, Unexamined Japanese Patent Publication No. 10-316,429 discloses that anatase TiO2 is suitable for use as a white electroconductive powder in the manufacture of semiconductors if it is doped with zinc or aluminum and its surface is coated with an electroconductive film of metal oxide. Karakitsou et al. disclose the use of cation doped TiO2 exhibiting improved performance as a photocatalyst in water cleavage in the Journal of Physical Chemistry, 1993, vol. 97, pages 1184-1189. Katayama et al. disclose the use of Nb2O5-doped TiO2 ceramics exhibiting improved humidity sensitivity.
This invention provides electrochemical cells exhibiting improved discharge capacity, especially on high current drains; and rechargeable electrochemical cells exhibiting improved cycle life.
One aspect of the invention is an electrochemical battery cell comprising an anode, a cathode, a separator between the anode and cathode, and an electrolyte, wherein at least one of the anode, cathode and electrolyte contains an n-type metal oxide additive.